Alkylidene ligands, such as CH2, CHR, or CR2, form M═C double bonds and their metal compounds are often called metal carbene complexes. The chemistry of late-transition-metal (i.e. groups VIII, IX and X of the periodic table) carbene complexes has recently received much attention, primarily due to the high catalytic activity of phosphine ruthenium carbene complexes in olefin metathesis. The most useful ruthenium carbene in these series is Grubbs' catalyst, (PCy3)2Cl2Ru═CHPh, bearing a benzylidene unit (Schwab P. et al., Angew. Chem. Int. Ed. Engl. 1995, 34, 2039; Schwab P. et al., J. Am. Chem. Soc. 1996, 118, 100, U.S. Pat. No. 5,831,108). Being highly active and remarkably tolerant to common functional groups, this compound found broad applications in both organic and polymer chemistry.
There are several synthetic approaches towards alkylidene complexes, with the ones utilizing the corresponding diazoalkane being the most popular and most general (Schwab P. et al., J. Am. Chem. Soc. 1996, 118, 100). However, the instability of diazo compounds and the safety issues involved in handling them seriously limit this method. From safety considerations it is also difficult to use such diazo compounds in industry.
Another recent approach, involving the reaction of precursors to unstable Ru(0) complexes with alkyl dihalides ((a) Belderrain, T. R.; Grubbs, R. H. Organometallics 1997, 16, 4001. (b) Olivan, M.; Caulton, K. G. Inorg. Chem. 1999, 38, 566) is limited by the difficult synthesis of the unstable Ru(COD)(COT) precursor.